1. Field of the Invention
The present invention relates to compositions useful for energy conversion and storage. More specifically, the present invention relates to the synthesis of manganese oxyiodides useful for high energy density battery and electrochemical capacitor (supercapacitor) applications.
2. Description of Related Art
Miniaturization in electronics and rapid advances in portable devices have created increasing demand for lightweight, compact, high energy density batteries (Scrosati, 1995). Lithium batteries offer higher energy density and longer shelf life compared to other rechargeable systems (Scrosati, 1995; Oyama et al., 1995). Although layered LiCoO.sub.2 may be used as a cathode in commercially available lithium-ion cells (Nagaura and Tazawa, 1990; Scrosati, 1992), Co is costly and toxic.
Manganese oxides are attractive in this regard because Mn is inexpensive and less toxic. Although spinel LiMn.sub.2 O.sub.4 has been pursued intensively as a cathode, its capacity fading on cycling due to Jahn-Telier distortion poses problems (Thackeray et al., 1983; Tarascon et al., 1994; Thackeray et al., 1998). Attempts to develop other crystalline manganese oxides have largely been unsuccessful because nonspinel oxides tend to transform to the more stable spinel phase on cycling (Gummow et al., 1993; Armstrong et al., 1996; Vitins et al., 1995). For instance, layered LiMnO.sub.2 tends to transform to a spinel phase and exhibits unsatisfactory capacity fading.
Recently, hydrated amorphous manganese oxides employing an aqueous medium have been reported (Xu et al., 1998). Although such oxides exhibit a high capacity, the capacity tends to decline to about 78% in 10 cycles. Because cyclability data is only available for 10 cycles, the stability of these water-containing cathodes upon prolonged cycling remains unclear.
Complex metal oxides used for energy storage devices are traditionally made by repeated grinding and firing of raw materials at elevated temperatures in order to overcome diffusional limitations. Such a "brute force", high-temperature approach often leads to unfavorable characteristics such as larger grain size, lower surface area, and an inaccessibility of metastable phases that may have unusual valences or atomic arrangements. These drawbacks have created interest in recent years in designing low temperature routes to synthesize complex materials (Stein et al., 1993).
It has been shown (Manthiram et al., 1994) that alkali metal borohydrides such as NaBH.sub.4 can be used effectively to reduce metalate ions (MO.sub.4).sup.n- (M=V, Mo and W) in aqueous solutions to obtain binary oxides M.sub.y O.sub.z and ternary oxides Na.sub.x M.sub.y O.sub.z. The method gave amorphous or nanocrystalline phases, which were often metastable, and the binary oxides such as VO.sub.2 and MoO.sub.2 obtained by this approach were found to be attractive as electrode materials for lithium batteries (Tsang and Manthiram, 1997; Manthiram and Tsang, 1996). However, these aqueous-based methods often give hydrated products, and complete removal of water while still maintaining an amorphous structure is difficult.